Nudge
This might be a long reply, but bear with me here
My idea for a "sporty" 102 goes something like this.
As you say "fill it with something", well i was thinking of firstly boiling the cylinder in degreaser to get rid of any oil residue that will almost certainly be in the metal itself,(we're talking microscopic levels here within the grain of the cast iron) then somehow braze some extra material into the combustion chamber to fill up any wasted/dead areas in order to resemble something like a square block cylinder head, thus getting nearer to what we might call a tighter "squish".(i know it's not the correct term but you understand what i mean here for a tighter clearance to the piston crown itself) I think a clearance gap of anywhere from 2-5mm should be more than ample for this, but seeing as the "extra"material can be removed or added at any time by machining (somehow) or re-brazing, there should be plenty of wriggle room to play with for safety's sake. A plasticine "mock up" could be used to acquire a better idea of what needs filling and by how much.There will be complications with the spark plug hole being positioned towards the lower portion of the combustion chamber, but i think it's workable. Ideally if i could get a cylinder with its plug hole more centrally drilled this could work out even better for what i have in mind. I have a number in my head of close to 100psi on the compression but this might need to be played with depending on what i find when i come to test it out for real. As you've mentioned already, keeping this "added material" firmly fixed into position and at the same time trying to avoid any warpage to the bore during this process is something that needs a lot of careful thinking about. Maybe some sort of heat resistant insert inside the bore could help with distortion but it still needs to have enough access to the combustion area of the cylinder during the brazing process. Of course it would be much easier if a 102 had a removable combustion chamber in the first place just like the square block models, but due to design this isn't going to happen so some ingenious ways of getting around this problem need to be overcome first if this idea is ever going to get off the drawing board. Ideas anybody? Even if the cylinder modifications seem like a non starter there are a few other places to look for improvement.
The use of an expansion pipe will probably be a massive step forward for a 102 aiding the cylinder filling for a more complete burn of fuel/air and hopefully a bit more power with less unburnt fuel/air mixture getting pushed straight out of the exhaust port before its had a chance to do its thing. It might also help to increase the engines maximum revs a bit too before it runs out of puff. My idea of hiding this expansion pipe inside the bone stock seagull exhaust tube might need a bit more thinking as i don't know how well a pipe inside a pipe is going to work when it comes to heat build up and how that heat will affect the way the expansion pipe will work within a desired rev range. I've seen vids and pics of other 102cc seagulls with "power pipes" fitted whichever way they seem to fit as they've probably pinched the pipe straight off the nearest 2 stroke motorcycle with similar displacements and they look ugly in my eyes but perfectly functional. In this instance those externally mounted power pipes are to some extent air cooled and probably quite noisy too. I'm thinking that if the "belly" of my homemade expansion pipe is similar in size to the ID of the stock seagull exhaust tube there won't be much room for the water delivery pipe to fit, so i could run this externally of the stock exhaust tube with a small cut out at the top of the tube so it still connects to the water fitting at the base of the cylinder. Once i've worked out how long the expansion pipe needs to be and in which specific rev range i want it to work i can better work out if this seagull will need to be a standard or longshaft to get the expansion pipe to fit inside the stock tube. Stealthy but without giving too much away
but at the same time looking like a typical 102 .The expansion pipe itself i can work out the basic dimensions, but the flange that will connect it to the port i'm having trouble working out the best way of transitioning from a rectangular shape (exhaust port) to a round shape (the header of the expansion pipe) has me a bit confused. A bit more reading and i'll have a better idea of where i'm heading with it.
Reed valves in a 102 i reckon could be a fantastic way of reducing any (my words again)"blow by" back out of the carb as they work like a one way valve basically, and so with a more intense filling of the crankcase with little or no "blow by" there's more fuel/air mixture previously drawn into the crankcase to get burnt and make power. In essence i'm trying to minimise the amount of wasted fuel/air that gets vented to atmosphere before its had a chance to do anything useful, something an old 102 needs a lot of help with i feel. To the best of my knowledge i'm not sure if anyone has experimented with reed valves in a 102 before but in principle i think it has a lot of potential. If i can get the humble 102 to breathe a lot better than they do already i think i could be onto something. Again, will i need to do something to the crankcases for a stronger vacuum (or what i call "draw"), i don't know- but as and when i come to test this beasty out for the first time i should get a better feel for where things need to be. Baby steps and all that.
Carbs are another area i've thought about. The later 400 amal's seem quite good for what they do in a stock application, but i'd like to see if there's anything a bit better with more adjustability for changing jet sizes for that perfect tune. Ideally for me something that has a mixture screw fitted to it for tweeking on the move like some of the cart racers do. Some mini bike carbs look quite good for this and they're fairly cheap to buy with plenty of jets around to tinker to your heart's content. Being able to richen or weaken fuel mixture to some extent whilst on the move i think could help with some fueling issues under hard continuous full throttle situations for extended periods.
Ignition systems are something that comes down to personal preference, CDI's to me at any rate seem to be a good way to go especially when used with a recoil starter for quick getaways from the start line
Variable ignitions could have a benefit here too, but baby steps and a healthy dose of caution here. With this proposed higher compression there's going to be more heat inside the combustion chamber, and it only takes milliseconds for some dodgy brazing to become unstuck and wreck a 102's piston and cylinder so this brazing mullarkey has to be spot on the first time around. With higher temperatures you run into problems with detonation and if the ignition timing is too far retarded/advanced it could prove deadly for a modified 102 being pushed higher than it was designed to go
There's a few "racers" on here that have had experience of pushing things too hard before now. I'm trying to learn from other folks mishaps but at the same time acquire a working knowledge of where those boundaries are with some of these old school seagull engines.
As someone once told me it's about "more bangs per minute" if you want to go faster. If i can get a humble 102 engine to run at higher revs without destroying itself in the process , make better power than it already does and stay reliable and in one piece without the need for constant rebuilding, i will have reached my goal and i will be a happy Jon. In practice it's probably not as simple as it sounds but what the heck, i'm up for a challenge. Importantly though, if i'm going to go ahead with this project of modifying a 102 engine it cannot have vast amounts of cash thrown at it. It has to be kept at a grass roots level and within the reach of most people's abilities when it comes to seagulls. There's a lot of people in seagull land that have access to well equipped workshops, not me i'm afraid and as a result i have to make do with what i've got by way of power tools and hand tools. Just keeping it real for a lot of us here that have an interest in going faster with their seagulls but don't want to spend a small fortune on something that could go horribly wrong very quickly.
It's not just the engine that will need some attention, the gearbox and prop will need some fettling as well if they're being pushed harder than they were designed to go. I have ideas here too, but a work in progress for later i think.
I've watched many videos of your boat Nudge and it seems to be fairly well set up and pretty fast for what it is.(how fast will it go on a good day?) Like me i bet you've wondered just how far you could reasonably push things with the dear old 102. Many folks like to fettle a square block seagull purely because they're much easier to modify, but the dear old 102 doesn't get anywhere near enough the attention i think they deserve. I'm very passionate about 102's as you can probably tell from previous posts. If anyone has got anything they'd like to share please feel free as all of what i've mentioned here is just ideas at the moment. Nothing has been put into practice yet.
The boat i'd like to build is a hydroplane NOT a hydrofoil. You must have seen this design before now (minimost or minimax sometimes called a sea flea) in varying lengths. I've also considered another design but can't find any plans for it yet,similar to a 3 point hydro with sponsons.Either way it needs to fit onto a car roof rack with ease and be lightweight.
https://www.youtube.com/watch?v=g4ovCCGhmaA
Going by the video this guys motor looks to be somewhat comparable to a modified seagulls power capabilities. Difficult to judge how fast this is going but 4 horsepower of Johnson's finest gets this little boat moving very well all things considered. If i can manage to get a 102 to do the same (maybe better and faster) i'll be more than happy with my achievements. Here's hoping anyway!
I hope this explains things in better detail Nudge
Jon
